def assign_variable(variable)
        env_var = variable.is_a?(EnvironmentVariable) ? variable : EnvironmentVariable.new(variable)
        all_variables.each { |existing_var| remove_variable(existing_var) if existing_var.key == env_var.key }
        @xml_element.add_element(env_var.xml_element)
        @all_variables << env_var